This description relates to semiconductor devices, and, more particularly, to transient voltage suppressor (TVS) diodes having a two-region base and methods of forming them using wide-band-gap materials.
At least some known punch-through transient voltage suppression (TVS) semiconductor devices include a uniformly doped base layer. The TVS device reaches breakdown when a depletion region of a reverse biased p-n junction reaches another p-n junction depletion region (or open base breakdown conditions are achieved). The blocking capability of the TVS device is defined by its base thickness and doping (amount of doping to be depleted to reach punch-through conditions). Punch-through limited breakdown design allows a simplification of the structure in such a way that no special edge termination is required and the TVS device can be fabricated using a mesa formation process. Although silicon carbide (SiC) can withstand up to about 3 Megavolt/centimeter (MV/cm), the breakdown strength of the mesa surface is usually 2-3 times lower, thus it is required to design the TVS device in a such way that an electric field in the TVS device is confined in the bulk region or is maintained less than a surface breakdown strength at all voltages up to breakdown.
The breakdown voltage and maximum electric field in a base of the TVS device at breakdown is at least partially dependent on a base thickness and/or doping. Generally, for a higher blocking voltage capability of the TVS device, the base is thicker and its doping is lower than for devices having a lower blocking voltage. Designing the TVS device with a specific breakdown voltage (BV) and low electric field (Emax) requires an even thicker base and lower doping.
For example, for an NPN or PNP TVS device with a breakdown voltage (BV) equal to approximately 600 volts (V), the SiC TVS device can have a base that is approximately 6 micrometers (μm) thick and has a doping concentration of approximately 2×1016 per centimeters cubed (cm−3). At breakdown voltage the electric field will reach approximately 2.2 Megavolts/centimeter (MV/cm). If for the same BV=600V, a lower electric field is required, for example, a maximum electric field (Emax)<1.1 MV/cm, the base region thickness would need to be increased to approximately 11 μm and its doping would have to be reduced to, for example, below 5.5×1016 cm−3. The drawback of “low electric field” TVS design is the requirement of a thick (especially at high BV) base region. The fabrication of such a high TVS mesa structure requires a deep etch process and a special mask protecting active area of the device during the etch process.
Increasing a thickness of the base layer to achieve a greater breakdown voltage has practical limits. Using a typical three layer NPN structure or PNP structure for any voltage, beyond a certain point, beyond a certain breakdown voltage, the increasing thickness of the base layer increases the resistance of the device itself, increases voltage clamping factor of TVS and thus worsens clamping capability of device.